This invention relates to the anodization of porous valve-metal anodes for capacitors and particularly to a method of forming a thin oxide layer on the outer surface thereof which has been anodized to a higher voltage than the rest of the anode to increase breakdown voltage adjacent the solid electrolyte layer.
It is known to anodize sintered valve metal electrodes in such a way that the oxide layer on the outer surface is formed (anodized) at a higher voltage than that on the inner surface of the sintered body. According to one method, described by Scheller et al in U.S. 3,415,722, issued Dec. 10, 1968, the sintered body is anodized at a normal voltage and impregnated with a material which is insoluble in the electrolyte, i.e., wax, stearin, anthracene, etc. The impregnant is extracted from the outer portion of the body, the anodization is continued at a higher voltage, and finally the impregnant is extracted from the inner portion of the body. The resulting structure has a higher breakdown voltage than prior art structures because of the thicker outer oxide layer. Alternately, Scheller et al propose to carry out the second anodization in a highly viscous electrolyte without impregnation, as such an electrolyte penetrates the inner surface slowly enough so that formation of the outer surface predominates.